1. Field of the Invention
The present invention relates to an electro-optic sampling probe, which is used for observing the waveforms of a test signal based on a change in the polarization state of a light pulse caused when the light pulse generated by a timing signal is input into an electro-optic crystal which is coupled with an electric field generated by the test measuring signal, and particularly relates to the electro-optic sampling probe provided with an improved optical system of the probe.
2. Background Art
An electro-optic probe is capable of observing waveforms of a test signal based on a change in the polarization state of a laser light caused when the light pulse generated by a timing signal is input into an electro-optic crystal which is coupled with an electric field generated by the test measuring signal. When the laser light is emitted in a pulsed mode, and when the test signal is used after sampling, the measurement can be executed that has a very high time resolution. An electro-optic sampling probe is developed by the use of the electro-optic probe utilizing the above phenomenon.
The electro-optic sampling probe (hereinafter, called EOS probe) has following advantages over the conventional probe using an electric probe, and thus such a probe is attracting attention.
(1) Measurement is easy, because a ground line is not necessary during measurement.
(2) Since the top end of the present electro-optic probe is insulated from the measuring circuit, a high input impedance is provided, which results in eliminating factors that disturb the conditions of the test point.
(3) The use of the light pulse allows carrying out wideband measurement reaching to the GHz order.
(4) Measurement can be executed for wiring that is too fine to be measured by direct contact with a metal pin by placing an electro-optic crystal in contact with an IC (Integrated Circuit) and by collimating the laser beam on the IC wafer.
The structure of the conventional electro-optic probe will be described with reference to FIG. 6. In FIG. 6, the numeral 1 denotes an IC wafer, which is connected with the outside through an electric source line and a signal line. The numeral 2 denotes an electro-optic element formed by an electro-optic crystal. The numeral 31 is an objective lens used for condensing a light incident to the electro-optic element. The numeral 41 is a probe body provided with a dichromic mirror 41a and a half-mirror 41b. The numeral 6a denotes an EOS optical module (hereinafter called an EOS optical system), and a fiber collimator 69 is mounted on one end of the EOS optical system.
The numeral 7 denotes a halogen lamp for illuminating the IC wafer for measurement. The numeral 8 denotes an infrared camera (hereinafter, called IR camera) used for confirming the positioning of the light condensed on the wiring of the IC wafer 1. The numeral 9 denotes an absorption stage for absorbing and fixing the IC wafer 1, and the absorption stage is capable of fine movement in the three directions of the x-axis, the y-axis, and the z-axis, which crosses each other at right angles. The numeral 10 denotes a standard table (partly omitted) to which the absorption stage 9 is fixed. The numeral 11 denotes an optical fiber for propagating the laser light that enters from the outside.
A light path of the laser light that enters from the outside is described with reference to FIG. 6. The light path of the laser light in the probe body 41 is shown by a reference symbol A.
The laser light incident to the EOS optical system 6a through the optical fiber is collimated into a parallel light beam by a fiber collimator 69, propagates through the EOS optical system 6a, and enters into probe body 41. Furthermore, the laser light propagates into the probe body 41, turned by 90 degrees by a dichromic mirror 41a, and condensed by an objective lens to the electro-optic element 2 at its surface that faces the IC wafer 1.
Here, a wavelength of the laser light entering into the EOS optical system though the optical fiber 11 is 1550 nm. In contrast, the optical properties of the above-mentioned dichromic color 41a allow transmission of 5% and reflectance of 95% of the light with a wavelength of 1550 nm. Therefore, 95% of the light emitted from the laser source is reflected and turned by 90 degrees.
A dielectric mirror is deposited on the surface of the electro-optic element that faces the IC wafer 1, and the laser light reflected at that surface is again collimated into parallel beams by the objective lens 31, returns to the EOS system. 6a passing along the same optical path, and entered into a photodiode (not shown) in the EOS optical system 6a. 
Next, a description is given on the light path of a light emitted by the halogen lamp 7 and a positioning operation of the IC wafer 1, when the positioning operation of the IC wafer 1 is carried out by use of the halogen lamp 7 and the IR camera 8. In FIG. 6, the symbol B denotes the light path of the halogen lamp 7.
The halogen lamp 7 used in this positioning operation emits light having wavelengths ranging from 400 nm to 1650 nm.
The light emitted from the halogen lamp 7 is turned by 90 degrees by the half mirror 41b, passes through the dichromic mirror 41a, and illuminates the IC wafer 1. The half mirror 41b used in this positioning operation yields reflected light with the same intensity as that of the transmitted light.
The IR camera 8 picks up an image of a part of the IC wafer 1 in the field of the objective lens illuminated by the halogen lamp 7, and the IR image is displayed on a monitor 8a. An operator executes fine movement of the absorption stage such that a measuring object, that is, the wiring on the IC wafer enters to a field of view.
Furthermore, the operator adjusts the position of the absorption stage 9 or the probe body 41 such that the laser light is condensed precisely on the surface of the electro-optic element 2 placed on the wiring of the IC wafer by confirming the laser light from the image of the IR camera 8 enters into the EOS optical system through the optical fiber 11, is reflected by the surface of the electro-optic element 2 placed on the wiring of the IC wafer 1, and passes through the dichromic mirrors 41a. 
In this operation, the laser light passing through the dichromic mirror 41a can be recognized by the IR camera 8, since the dichromic mirror can transmit about 5% of light in the wavelength range of the laser light.
Here, a measuring operation of test signals by use of the EOS probe shown in FIG. 6 is described.
When a voltage is applied on the wiring of the IC wafer, the electric field is applied to the electro-optic element 2, causing a change in its refractive index due to the Pockels effect. Thereby, when the laser light enters into the electro-optic element, reflected at the surface of the electro-optic element placed facing the IC wafer, returns the same light path, and exits from the electro-optic element, the polarizing state of the laser light changes. After being subjected to the change of the polarizing state, the laser light enters again on the EOS optical system 6a. 
Since the polarized state of the electro-optic element in the EOS optical system has been changed, the intensity of the light incident to the EOS optical system is changed in accordance with the change of polarized state, the change of the light intensity is converted into an electric signal after being received by a photodiode, and the electric signals applied to the IC wafer 1 can be measured by processing the signals from the photodiode.
There are some ICs such as light switches which are operated by irradiation of excitation light, that is, light for excitation on the front surface or the rear surface of the IC wafer. However, the problem arises in the conventional electro-optic sampling probe that the measurement of the electric signals can not be simultaneously carried out while the excitation light is irradiated on the front or the rear surfaces.
In order to solve the above problem, Japanese Unexamined Patent Application, First Publication No. Hei 10-340824, discloses an electro-optic sampling probe, capable of irradiating the excitation light from both surfaces of the IC wafer without displacing the IC wafer and capable of measuring electric signals while irradiating sampling light from surfaces of the IC wafer.
However, the problem still arises in the above electro-optic sampling probe that the excitation light can not simultaneously enters on a plurality of light receiving portions provided on the IC wafer 1. If the spot size of the excitation light is enlarged so as to simultaneously irradiate the plurality of light receiving portions, the excitation light irradiates the surface area outside of the light receiving portion, which leads to an inaccurate measurement. In addition, the problem still remains that the enlarged spot irradiation does not allow sequential time measurement.
It is therefore an object of the present invention to provide an electro-optic sampling probe capable of entering the excitation light simultaneously on a plurality of light receivers provided on the IC wafer.